Car manufacturers make a big deal out of the purported fuel consumption of their cars. Since April 2009, it’s been a legislative requirement for car makers to attach an official fuel consumption label to any new cars in the showroom. And that label must display not just one but three different fuel consumption figures (bureaucrats...) as well as CO2 output in grams per kilometer.

Official fuel consumption label - a legislative requirement on all new cars in showrooms

So how come, after all this legislation and regulatory waffle aimed at stopping manufacturers from fudging the figures, in practice, cars still never seem to achieve the fuel economy specified in the glossy brochures, or the official label.

Plenty of customers use those numbers in their decision to buy a particular car. Then, out on the road, it always seems more thirsty than advertised.

So, in this situation you approach the dealer. You probably get fobbed off. Your intelligence gets insulted. Then you get angry. It’s understandable. It seems like the car is defective, and nobody’s taking you seriously.

Those official fuel consumption numbers are part of the Australian Design Rules, or ADRs. In other words, the official compliance standards for new cars in Australia. The fuel economy standard is called ADR 81/02.

Car companies do those fuel tests in government-audited independent laboratories. They aren’t driven on roads; they’re connected to machines called dynamometers. Every variation of every model gets tested separately. Two highly standardized tests are conducted – they’re called the Urban and Extra-urban tests.

(This is what happens when propeller-heads get involved in anything. They can’t even use words like ‘city driving’ and ‘open road’. Is it any wonder the tests themselves are out of step.)

How the official fuel test works; speed Vs time

Together, both tests take 20 minutes. In the 13-minute Urban test, the car averages 19km/h. It spends a total of six minutes stopped. It actually stops 12 times during that test and peaks at 50km/h, four times. In the seven-minute Extra-urban test, meant to simulate highway driving, the average speed is 63km/h and the maximum is 120. A total of 40 seconds is spent stopped.

The tests can’t be faked, so nobody’s cooking the books. The tests are repeatable, standardized and tightly controlled. But they’re about as unlike actual driving as you can get.

On one hand, they’re quite good for comparing different cars you might be thinking about buying. In other words, good for relative comparisons. Is the manual more efficient than the auto? Is the diesel more efficient than the petrol? Is the Mazda3 more efficient than the Hyundai i30? They’re good for that kind of comparative shopping.

Unfortunately they’re lousy as absolute indicators of how thirsty your particular car is going to be. If you use those numbers for that, you will be disappointed – every time.

Car companies know fuel consumption is important to you, so they proclaim these official fuel test numbers loud and clear – they’re pumped up big and bold in all the marketing claims, and the disclaimers are hidden in the small print you’d need a magnifying glass to read.

To recap: the regulations are out of step with reality, and car companies trumpet the numbers like there’s no tomorrow.

You could easily buy into those numbers – only to feel gutted when the car fails to deliver them, over and over and over.

(What they should have done is design the tests to over-estimate fuel consumption – then every car would under-consume out there on the road, and everyone would live happily ever after. Just like the Brady Bunch.)

As a rule of thumb, you should take the official ‘combined cycle’ fuel number, and add about 30 per cent. That’s what your new car is actually likely to consume – if you’re a normal driver. More or less. More, if you’re a hoon.

At least the official fuel numbers are not outright lies. They’re a kind of institutionalized misrepresentation of actual fuel economy. Don’t let it form the foundation for a pathological obsession – if you’re thinking: lawyers at 10 paces, you probably don’t have a leg to stand on.

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